Suppose you borrowed $$21,000$ at a rate of $9\mathrm{.}4\%$ and must repay it in $4$ equal installments at the end of each of the next $4$ years. How large would your payments be$?$ a$.$ $$5,977\mathrm{.}19$ b$.$ $$6,539\mathrm{.}04$ c$.$ $$7,224\mathrm{.}00$ d$.$ $$4,565\mathrm{.}04$ e$.$ $$7,520\mathrm{.}19$